The invention relates to new thermosetting powder coating compositions. Specifically, the invention relates to new acid functional acrylic/polyepoxy powder coating compositions. More specifically, the invention relates to new acid functional acrylic /polyepoxy powder coating compositions for coatings on heat sensitive substrates such as wood substrates to produce thereon by a low temperature cure a finish with high hardness and/or controllable gloss.
It is well known that powder coating is a low polluting, cost effective method to provide high quality finishes on substrates such as metal substrates and therefore, is used more and more to replace liquid finishing in various industries. Traditionally, powder coatings are used to coat metal objects, e.g., in the automotive and appliance industries. Metal objects are very good electric conductors and can be heated to high temperature. Thus, conventional powder coating compositions are easily applied onto metal substrates by any of the electrostatic deposition techniques known in the art to form a fine finish. The coated metal substrates are normally cured at elevated temperatures ranging from 300xc2x0 F. to 450xc2x0 F. Since high temperatures do not significantly distort metal substrates, heat sensitivity of the work piece is generally not a concern. There has been well developed in the art powder coating compositions that are cured at high temperatures to provide high performance coatings on metal substrates.
More recently, efforts have been made to apply powder coatings to non-metal substrates, known as heat sensitive substrates, e.g., plastics, pre-assembled products and wood products. Examples of wood products include household furniture, office furniture, and construction components such as doors, siding, widow frames, shelving and molding. While some plastic articles may be manufactured with the appropriate color and may not need coatings, some other plastic products and especially many wood products must be coated for decorative or protective purpose or both. Although some success has been achieved in coating non-metal substrates, problems remain, especially with coatings on wood substrates due to the distinct natural difference between metal substrates and wood substrates.
Naturally, wood substrates contain more moisture and volatile compounds. The moisture and the volatile compounds are readily released as gasses during the high temperature cure process associated with traditional powder coatings on metal substrates. This outgassing disrupts the surface of the coating material, resulting in pinholes and other defects in the coating surface, thereby damaging the appearance of the coating. Therefore, conventional high cure temperatures are not usable for coating powders on wood substrates. Presently, one approach to avoid pinhole and defects in the coating surface is to provide a textured coating having, e.g., a grainy texture, or a coating with an orange peel appearance that mask the underlying non-uniformity of the substrate. Another approach to eliminate outgassing is to develop a low temperature cure coating powder system to lower the cure temperature and/or shorten the cure time. However, the lower the cure temperature and/or the shorter the cure time are, the more difficult it is to achieve high performance. Likewise, the lower the cure temperature and/or the shorter the cure time are, the more difficult it is to control gloss, especially to achieve aesthetically desired low gloss. Further, the lower the cure temperature and/or the shorter the cure time are, the more difficult it is to process the powder coatings as traditional process temperatures are in the same range as the targeted cure temperatures. Higher performance coatings as defined by toughness and durability tests like hardness, scratch, mar, chemical resistance and impact can be challenging with traditional coating powders cured at high temperatures. Again the challenge becomes greater as the cure temperature is lowered or the cure time is shortened.
Various powder coating compositions used on either or both metal and wood substrates in the relevant art are described in the patent literature.
U.S. Pat. No. 6,022,927 discloses an acrylic powder coating composition used on metal and wood substrates. The composition contains basically an acid functional acrylic resin; an adduct of an acid functional polyester and a polyepoxy (e.g., trigylcidylisocyanurate (TGIC)); and a curing agent that is also TGIC to provide orange peel textured surfaces on both metal and wood substrates. It is specifically taught that a coating powder prepared by simply mixing the carboxyl functional acrylic resin, the curing agent (TGIC) and the polyester does not provide a coating with desired physical properties. Only if the curing agent (TGIC) and the polyester are adducted prior to forming the coating powder, can the coating with desired properties be produced. The cure temperature may be reduced to 300xc2x0 F. or below upon addition of a cure catalyst. 60xc2x0 gloss tested on metal substrate is higher than 75. Pencil Hardness of the coating surface, tested on metal panel, is from F to 2H.
U.S. Pat. No. 5,907,020 discloses a thermosetting powder coating system, based on a mixture of an extrudate of a catalyzed self-curing epoxy resin and a low temperature curing agent. The epoxy resin is first extruded with a catalyst, then the extrudate is mixed with a low temperature curing agent to form the coating powders. The low temperature curing agent is an epoxy adduct of an imidazole catalyst or an epoxy adduct of an aliphatic polyamine, to provide low gloss coatings on metal and wood substrates. The cure temperature may be 290xc2x0 F. or below. 60xc2x0 gloss tested on steel panel varies from 15 to 80.
U.S. Pat. No. 5,721,052 discloses a thermosetting powder coating composition based on Bisphenyl A type of epoxies, an imidazole catalyst and a texturing agent, for forming a grainy textured finish on both metal and wood substrates. The cure temperature may be about 350xc2x0 F. or below. 60xc2x0 gloss tested on metal panel is below 6. Pencil Hardness, tested on metal panels, is from H to 2H.
All the powder coating compositions disclosed, however, do not provide coatings on heat sensitive substrates, especially wood substrates with the advantageous combination of a low cure temperature, controllable gloss, and very high hardness.
Therefore, there is a need for a new powder coating composition that can be coated by means of traditional powder coating application methods on heat sensitive substrates, especially wood substrates and cured at low temperature to produce a finish with controllable gloss and very high performance such as hardness without damaging the substrates.
The invention provides a new powder coating composition in particulate form consisting essentially of a mixture of:
a). about 10% to 90% by weight, relative to (a) plus (b), of carboxylic acid functional acrylic resin(s);
b). about 10% to 90% by weight, relative to (a) plus (b), of polyepoxy resin(s);
c). a catalyst in an amount sufficient to cure the composition at a temperature about 300xc2x0 F. or below for about 30 minutes or less; and
d). optionally, a flexibilizing agent.
In another aspect, the invention provides a new powder coating composition comprising:
a). about 10% to 90% by weight, relative to (a) plus (b), of carboxylic acid functional acrylic resin(s);
b). about 10% to 90% by weight, relative to (a) plus (b), of polyepoxy resin(s); and
c). a catalyst in an amount sufficient to cure the composition at a temperature about 300xc2x0 F. or below for about 30 minutes or less,
wherein the composition provides a finish having a pencil hardness of at least 3H.
In yet another aspect, the invention provides a new powder coating composition comprising:
a). about 10% to 90% by weight, relative to (a) plus (b), of carboxylic acid functional acrylic resin(s);
b). about 10% to 90% by weight, relative to (a) plus (b), of polyepoxy resin(s);
c). a first catalyst which functions primarily as an epoxy homopolymerization catalyst; and
d). a second catalyst which functions primarily as an epoxy/carboxylic acid reaction catalyst.
The new powder coating-compositions of the invention are readily applied, e.g., by means of electrostatic deposition or fluidized bed methods, to various substrates, especially heat sensitive substrates, e.g., plastic or wood substrates to provide finishes having controllable gloss and high performance including very high hardness.
The coatings of the invention may be cured at a temperature of about 300xc2x0 F. or below for about 30 minutes or less to form coating films on substrates, especially wood substrates, thereby, diminishing outgassing from the substrates without damaging the substrates. The cured coatings of the invention may produce 60xc2x0 gloss of from about 2 to 95 and pencil hardness of from about F to 9H.
Particularly, the new powder coating compositions of the invention are readily coated by electrostatic spray and cured on heat sensitive substrates, especially wood substrates at a temperature of about 280xc2x0 F. or below for about 20 minutes or less, to produce a finish having an aesthetically acceptable 60xc2x0 low gloss of from about 5 to about 60, preferably, of from about 10 to about 40, and pencil hardness of from H to 9H, preferably, from 3H to 9H.
In yet another aspect, the invention provides an article comprising a heat sensitive substrate coated on at least one surface of the substrate with any of the aforesaid new powder coating compositions of the invention and cured at a temperature of about 300xc2x0 F. or below for about 30 minutes or less to produce a decorative and/or protective finish with controllable 60xc2x0 gloss of from about 2 to 95 and controllable performance regarding chemical resistance, hardness, etc., depending on application.
In particular, the article of the invention comprises a wood substrate coated on at least one of the surfaces of the substrate with any of the aforesaid new powder coating compositions of the invention and cured at a temperature of about 300xc2x0 F. or below for about 30 minutes or less to produce a finish having 60xc2x0 gloss of from about 5 to 60, preferably from about 10 to about 40, and pencil hardness of from F to 9H, preferably, from H to 9H, and more preferably from 3H to 9H.